Polymer beads are often made by suspension polymerization in aqueous medium.
Sometimes, a "failure" of suspension occurs. When this happens, the hydrophobic materials tend to agglomerate and form a solid mass. This necessitates a shut-down of the equipment and a cleanout -- often by chipping and hammering the mass of polymer from the interior of the reactors. This type of failure consumes time in cleaning and is costly, especially in lost production capacity.
If the suspension polymerization is run in an open reactor, the onset of suspension failure can be visually noted and the instant addition of stabilizers can be undertaken usually in time to avoid the failure.
However, if the suspension polymerization is run in a closed reactor, visual monitoring cannot be done. The need for a monitoring system to allow one to "see" the onset of suspension failure in closed reactors has been long recognized.